This invention relates to a hot cathode of an X-ray tube and more particularly to a hot cathode of the kind having a thermoelectronic emitter supported by a heating element.
It is known to use lanthanum hexaboride (LaB6) as the material of a thermoelectronic emitter of a hot cathode of an X-ray tube. The lanthanum hexaboride may constitute a hot cathode as it is as disclosed in FIGS. 1 and 14 of Japanese Patent Publication 10-321119 A (1998) or may be supported by a heating element made of carbon or the like to complete a hot cathode as disclosed in FIGS. 9 and 10 of the same Japanese Patent Publication 10-321119 A (1998). The present invention is directed to the latter case, i.e., a thermoelectronic emitter is supported by a heating element.
The hot cathode of the kind having a thermoelectronic emitter, which is made of lanthanum hexaboride and supported by a heating element made of carbon, can be produced by the steps of making grooves on the heating element, filling the grooves with lanthanum hexaboride powder and sintering the powder as disclosed in Japanese Patent Publication 2001-84932 A.
However, in case of producing a narrow thermoelectronic emitter, for example, 10 mmxc3x970.5 mm, by sintering lanthanum hexaboride powder as mentioned above, it has been reported that a certain problem occurred. The report said that when the sintered hot cathode had been used for a long time to generate X-rays in an X-ray tube, the filament current of the X-ray tube, i.e., the current flowing from one end of the hot cathode toward the other end, showed a large hunting phenomenon and thus the current was uncontrollable. The filament current is normally controlled to become, for example, 1.2 Axc2x10.5 A. If the uncontrollable phenomenon occurs, the current departs from the normal range far away and can not be restored, so that the control circuit is terminated and the X-ray generation stops and thus the X-ray tube can not be used. Once the uncontrollable phenomenon occurs, the filament current can not be controlled, requiring the hot cathode exchange.
Inspecting the hot cathode which has become uncontrollable, the following cause was seen. Observing, with a microscope, the surface of the thermoelectronic emitter which is made of lanthanum hexaboride and has a plane size of 10 mmxc3x970.5 mm and a thickness of 0.3 mm, several cracks were found. It was found also that all of the several hot cathodes which have become uncontrollable showed the similar cracks. Even when the particle size of the lanthanum hexaboride powder was changed, the tendency to cracks was unchanged although with a difference in degree. Of course, the hot cathode right after the sintering of the lanthanum hexaboride powder shows no crack. The thermoelectronic emitter is supposed to have random cracks after receiving any physical or thermal shock in the course of X-ray generation.
It is an object of the present invention to provide a hot cathode of an X-ray tube of the kind having thermoelectronic emitter supported by a heating element, in which no crack occurs on the thermoelectronic emitter.
Observing a certain hot cathode having cracks, several cracks were seen at intervals of several millimeters on a narrow thermoelectronic emitter. Then, further observing other several hot cathodes having cracks and measuring the distances between neighboring cracks, it has become clear that almost of the distances were more than three millimeters. Accordingly, we have produced an improved hot cathode in which a thermoelectronic emitter was divided into plural regions arranged in a straight line and the length of each region was less than three millimeters with the total length of the emitter being about ten millimeters, and then conducted a running experiment with X-ray generation. As a result, it was found that an uncontrollable phenomenon in filament current did not occur and the hot cathode taken out after the experiment showed no crack, which has been ascertained by observing with a microscope. On the basis of this experiment, the present invention has been developed in which the length of each emitter region is less than three millimeters and plural emitter regions are combined with each other to constitute a thermoelectronic emitter with a desired length so as to obtain a hot cathode with no danger of cracks.
Accordingly, the present invention provides a hot cathode of an X-ray tube of the kind having thermoelectronic emitter supported by a heating element, in which the thermoelectronic emitter is comprised of plural emitter regions separated from each other, each of the emitter regions having the largest measure less than three millimeters. The thermoelectronic emitter shows no crack and the filament current is stabilized.
It is noted that the xe2x80x9clargest measurexe2x80x9d of an emitter region stands for the largest value among all distances between any one point on the emitter region surface and any another point on the same emitter region surface. For a narrow emitter region, the largest measure is approximately the same as its length. For a circular emitter region, the largest measure is the same as its diameter. The present invention may be applied to not only narrow emitter regions but also emitter regions of any shapes. Even if the emitter regions have any shapes, no crack occurs as long as the largest measure is less than three millimeters.